This application claims the priority of German application No. 196 51 458.4, filed in Germany on Dec. 11, 1996 and German application No. 197 25 678.3, filed in Germany on Jun. 18, 1997, the disclosures of which are expressly incorporated by reference herein.
This invention relates to an apparatus for obtaining oxygen from the air. More particularly, this invention relates to an alternating pressure apparatus for obtaining oxygen from the air, which has at least one adsorber containing synthetic zeolite to which an air blower for the adsorption air and one vacuum pump stand for the desorption of the adsorber are connected for alternate operation, the vacuum pump stand having two vacuum pumps in tandem. The invention furthermore relates to a method for the operation of such an alternating pressure apparatus.
An alternating pressure apparatus of the type described above, which serves, however, for the production of nitrogen from the air, is described for example in DE-A-31 46 189. In the vacuum pump stand of the known alternating pressure apparatus, between the two vacuum pumps there is disposed an intermediate cooler configured as a heat exchanger, which operates dry, in contrast to the cooling by water injection that is generally used, and which cools the air compressed by the first vacuum pump to the extent that no overheating occurs in the second vacuum pump. The second vacuum pump is provided with preliminary inlet cooling. The alternating pressure apparatus according to that disclosure operates with adsorption pressures between 1 bar and 2.5 bar and desorption pressures of 85 mbar.
In alternating pressure apparatus for the production of oxygen, however, the cooling of the air by water injection has become widespread, because it was assumed that the vacuum pump stand with the operation of the two vacuum pumps can be virtually isothermic and therefore especially energy efficient.
The injection of water into the oxygen-depleted air aspirated by the vacuum pump stand assumes that extremely clean processed water free of minerals is used so as to avoid malfunction of the vacuum pumps. Often, however, no water or no suitable water is available for injection, so that apparatus using water injection are out of the question and somewhat less energy-efficient apparatus are given preference, like those described in the above-mentioned DE-A-31 46 189. But since such apparatus require water for cooling the air, their employment involves difficulty in many places. Dispensing with cooling water or injection water has not been possible heretofore, however, because the vacuum pumps, configured as rotary piston pumps, tolerate a maximum temperature of about 125.degree. C. to 130.degree. C., and in the known apparatus the aspirated air, unless cooled, would be heated by the compression in the vacuum pump stand to about 200.degree. C. after the second stage.
The present invention solves the problem of designing an alternating pressure apparatus of the kind referred to above such that it will be able to operate in an energy efficient manner without water injection or water cooling and at temperatures safe for rotary piston pumps. Furthermore a method for operating such an alternating pressure apparatus is provided.
The problem first mentioned is solved by the invention in that the second vacuum pump in the direction of flow is a rotary piston pump with preliminary inlet cooling, having cooling air inlet ports for the defined inflow of outside air of atmospheric pressure, that the inlet connection of the second vacuum pump, without the use of gas cooling by water injection or by heat exchanger has a connection to the outlet of the first vacuum pump, and that the vacuum pump stand is provided with control systems for producing a desorption pressure between 300 mbar and 500 mbar.
Since according to the invention higher desorption pressures are used than in the state of the art, relatively low compression work occurs in the second vacuum pump operating against the atmosphere. Due to the admixture of air from the atmosphere, the heat thus produced causes little or no temperature rise, so that the vacuum pump stand can operate entirely without water injection or water cooling. Surprisingly, measurements on a constructed alternating pressure apparatus according to the invention showed that, at the same oxygen yielding power as in known alternating pressure apparatus, the energy required is even lower.
An especially low energy requirement is achieved if, according to an advantageous embodiment of the invention, the cooling air inlet ports of the second vacuum pump are of such dimensions and positions that the ratio of the total mass flow of the aspirated mass flow of the first vacuum pump and of the cooling air mass flow to the cooling air mass flow at the start of the desorption cycle drops rapidly from a high level to 33 and during the desorption cycle slowly drops to 1.5 by the end.
It is important to the cooling principle according to the invention that, in the rotary piston pump operating as a vacuum pump, the rotary pistons block the connection to the inlet connection before the working pump chamber receives a connection to the atmosphere and therefore the air can flow in and perform compression work. This can be achieved economically because, according to another embodiment of the invention, the cooling air entry ports in the second quadrant begin not before 36.degree. and end before 900.degree..
The fundamental frequency of the pulsations can be doubled, so that simple pulsation dampers can be used if the cooling air inlet ports are connected by a common line to the atmosphere.
Since the air blower for the adsorption air and the vacuum pump stand produce very bothersome noise in operation, these parts of apparatus are usually disposed in a noise-suppressing box container. Therefore a great deal of expense is involved in cooling with a positively driven air stream. This expense can be kept comparatively low if, according to another embodiment of the invention for cooling the apparatus, a blower of an air cooler necessary for the adsorption air is provided, and if a cooling air stream is guided along the vacuum pump stand and the adsorption air blower to the blower.
An excessively low temperature of the air entering the adsorber can be prevented simply by providing the blower with a controlled-speed fan motor. In this manner the amount of cooling air can be reduced at low external temperatures by reducing the speed of the blower.
The alternating pressure apparatus is very economically constructed if a radial blower with a controlled throttle in its intake line is provided as the first vacuum pump.
It is also desirable if only one line common to the positive pressure and the negative pressure runs from a distribution line or manifold into each adsorber, if an air valve and a vacuum valve are inserted into the distribution line on each side of the point where the lines are connected to the distribution line, and if the air blower or the vacuum pump stand is connected to the two outer ends of the distribution line and a line from the vacuum pump stand leads into the middle of the distribution line between the air valves. In this manner it is made possible with very little expenditure on piping and valves to connect the air blower to the inlet of the vacuum pump stand when operating at partial load. Thus the pressure difference of the vacuum pump stand is reduced to about 200 mbar, so that without the expense of a complex bypass for the vacuum pump stand it is possible to operate at partial load with a correspondingly lower energy demand.
Back flow of oxygen from the buffer tank through the adsorbers and then through the vacuum pump stand to the atmosphere when running at idle, when the air blower is connected to the inlet of the vacuum pump stand, can very easily be prevented by providing a shut-off in the product delivery line.
The shut-off is of very simple construction and does not need to be operated in harmony with the adsorber outlet valves if it is a check valve opening toward the storage tank.
When running at partial load it is possible to operate entirely without the first vacuum pump in the form of a radial blower, and thus save much energy, if, according to another embodiment of the invention, the first vacuum pump in the form of a radial blower has a bypass line connecting its suction line directly to the second vacuum pump in the form of a rotary piston pump, and containing a check valve port toward the second vacuum pump.
For further improvement of the energy balance it is helpful if, to evacuate the first vacuum pump, configured as a radial blower, an auxiliary pump is provided. Thus the radial blower can run partially loaded without having to perform work.
The vacuum pump stand is able, independently of the air blower, to aspirate air from the atmosphere if an air inlet line having a check valve closing toward the atmosphere is inserted into the line connecting the air blower to the adsorbers. Such a configuration, however, is necessary only in exceptional cases.
The second problem referred to, namely the creation of a method for operating an alternating pressure apparatus with the above-described features, is solved by the invention in that a rotary piston pump with preliminary inlet cooling is used, into the cooling air inlet ports of which outside air at atmospheric pressure is allowed to flow at a controlled rate, that the gas leaving the first vacuum pump is fed to the inlet of the second vacuum pump without the interposition of cooling by water injection or by a heat exchanger, and that the vacuum pump stand is operated at a desorption pressure between 300 mbar and 500 mbar.
With a method of operation of this kind it is possible to produce oxygen without cooling water or water injection and with an energy consumption decidedly lower than in the known apparatus.
It is especially good from the energy viewpoint if in the second vacuum pump the ratio of the total mass flow of the aspirated mass flow of the first vacuum pump plus the cooling air mass flow to the cooling air mass flow falls rapidly from a high level at the start of the desorption cycle to 33, and within the desorption cycle slowly sinks to 1.5 by the end.
For operation at partial load, the energy requirement will be lower corresponding to the desired lower oxygen amount if, during the partial load operation characterized by low oxygen removal, the adsorption pressure is increased above the adsorption pressure at full-load operation, and when a set top adsorption pressure is reached, the air blower for the adsorption air is switched ahead of the two-stage vacuum pump stand.
Partial load operation can be terminated with especially little effort if the pressure at the storage tank or buffer tank is monitored, and when it falls below a set level, the air valves and vacuum valves are switched back to cyclical adsorption and desorption. The pressure drop that is measured indicates that oxygen uptake is again increasing.
It has been possible to learn by experiment that energy consumption is especially favorable if, according to another embodiment of the method, an adsorption pressure between 0.7 bar and 1.8 bar, preferably between 0.9 bar at the beginning and 1.5 bar at the end of the adsorption is selected along with a desorption pressure between 350 mbar and 400 mbar, and if the starting pressure for the adsorption and desorption after the purging phase and the pressure build-up phase amounts to a minimum of 700 mbar and a maximum of 950 mbar.
After a changeover to building up pressure, in order to be able to achieve a build-up to adsorption pressure in an adsorber, according to another embodiment of the invention the pressure is throttled by means of a throttle in the product delivery line behind the buffer tank.
It is also beneficial to the operation of the alternating pressure apparatus if the throttle is followed by a buffer tank and the capacity of the latter is such that the pressure in the buffer tank varies between 1.2 bar and 1.5 bar within an adsorption cycle.
The adsorption air blower and the vacuum pump stand are cooled by a positively guided air stream, without the danger of an excessively low input temperature in the adsorber at low outside temperatures, if a blower of an air cooler required for the adsorption air is used for cooling the parts of the apparatus, if a cooling air stream is carried along the vacuum pump stand and the adsorption air pump stand to the blower, and if the blower is controlled by a frequency-controlled fan motor such that the temperature of the air entering the adsorber does not fall below a minimum level. Experience has shown that the oxygen concentration is considerably reduced if the adsorption temperatures fall below 20.degree. C. By the procedure of the invention, decreasing outside temperatures can be compensated by reducing the volume of the cooling air stream.
If the outside temperature is so low that reducing the speed of the cooling air blower is insufficient to maintain adsorption temperatures above 20.degree. C. it is possible according to an embodiment of the invention to use the hot air put out by the vacuum pump stand to raise the temperature of the air from the air blower. Thus, a throttle valve is installed ahead of the air blower so that some cold air is aspirated from the container housing the vacuum pump stand, and another air stream preheated by the vacuum exhaust through a heat exchanger is aspirated! directly at the exhaust air exit. By this the temperature level after the air blower is raised accordingly, so that the 20.degree. C. adsorption temperature can be maintained by controlling the speed of the motor driving the cooling air blower. This preheating is possible because the second vacuum pump according to the invention operates without water injection and thus operates at higher temperatures than in the state of the art.